We cannot do anything for the display problems in Dreamweaver.This boils down to an a Dreamweaver issue really.
You may be able to get the design view in Dreamweaver to display a little better by adjusting your screen resolution.
As far as previewing the page in firefox, there is a known bug in Dreamweaver with loading pages into firefox on Mac OS when pressing the F12 key or selecting the Preview in browser command.
I have this issue on my system as well.
I work around is to use Safari as your browser and copy the address from the safari address bar and paste it into the fire fox address bar.